Improvers for photographic emulsions



Patented July 16, 1946 IMPROVERS FOR PHOTOGRAPHIC EMULSIONS John David Kendall, Douglas James Fry, and

James Douglas Brooks, Ilford, England, assignors to Ilford Limited, Ilford, Essex, England, a

British company N 0 Drawing.

Application November 12, 1943,

Serial No. 510,040. In Great Britain December 21 Claims.

This invention relates to compounds which serve as improvers for photographic silver halide emulsions and particularly to compounds which, when added to the emulsion, or to the developer used in processing the emulsion, or otherwise applied to the emulsion before development, improve the quality of the developed photographic image obtained.

On exposure and development of a normal photographic film, plate or paper bearing a silver halide photographic emulsion there is produced, in addition to the silver image, a general deposit of metallic silver, the distribution and density of which bears no direct relation to the variations in luminosity of the subject. This overall deposit is referred to as chemical fog or development fog. Its efiect is to increase the general density of the negative obtained and to depress the contrast of the image in the least exposed parts. In general the fog density produced is greater in the case of high speed emulsions than in slow emulsions, and it is invariably greater the more complete the development, e. g. the higher the temperature of development, and when Vigorous developers are used.

Moreover, the degree to which an emulsion produces fog on development usually increases with the age of the emulsion, so that photographic emulsions which have been kept a long time, especially in warm or humid conditions, and emulsions which have been kept a long time in the molten condition, may produce images associated with'a heavy fog density. Many compounds have been suggested in the past for use as additions to emulsions or to developers which tend to reduce the formation of fog, but almost all of them, if reasonably effective as antifogging agents, have caused a serious reduction in the speed of the photographic emulsion.

Further, in the production of photographic images in silver halide photographic emulsions, and particularly in the case of emulsions consisting primarily of silver chloride, the maximum image density obtainable is not always so great as may be desired. Moreover, the common prac- 'tice of hot-glazing photographic prints tends to affect adversely the image colour, giving a poor quality black known as the "bronzing effect, and

also reduces the effective maximum density ob-.

with a small quantity of a 5-mercapto tetrazole of the genera1 tautomeric formulae:

where R is a hydrogen atomor a hydrocarbon substituent group. (which itself may be substituted or unsubstituted), at a stage not later than the development of the emulsion, has a considerable improving effect on the quality of the developed photographic image in respect of the characteristics referred to above.

The compounds are preferably included in'the emulsion during its manufacture, or in the developer used, but any other methodof bringing the compounds into operative. contact with the emulsion may be used, for example the compounds may be included in subcoat, supercoat or backing layers which are, or become, in contact with the emulsion, or they may be included in the wrapping materials used, or they may be applied to the emulsion in solution before the emulsion is developed.

The compounds of this invention are not equally effective in all respects, and it has been found that whilst all of them serve to reduce development fog to some extent and also serve to cause an effective increase in maximum image density, especially after hot glazing, nevertheless, some compounds of the series are better for one purpose than for the other. Thus, for example, where the primary object is to stabilise the emulsion, or to reduce the fog density of the developed image, those compounds of the above general formulae in which R is an aryl, substituted aryl, aralkyl or cycloalkyl group are preferred. The compounds in which R is a hydrogen atom or an alkyl group, though efiective, are generally less satisfactory from this standpoint. The aryl, substituted-aryl, aralkyl and cycloalky1 compounds are effective anti-fogging agents, however brought into operative contact with the emulsion, but they are preferred for use in developers or in the emulsions. The compound 1-phenyl-5-mercapto-tetrazole is typical of this class, and it is found to preserve high speed photographic emulsions against excessive fog over long periods of time and also toenable development to be effected with vigorous developers or at high temperatures. Thus, these compounds are of particular value as emulsion or developer additions in cases where the emulsions or developers are to be used in tropical conditions or, in the case of emulsions, where the emulsions are to be kept for a long time in the molten condition. These compounds further present the advantage that their use in concentrations sufficient to cause a useful fog reduction does not seriously reduce the effective speed of the emulsions in which they are incorporated, or to which they are applied in developer solution, and in some cases the compounds actually increase the effective speed.

All the compounds, whatever the nature ofithe l-substituent (if any), have a valuable effect in increasing the maximum' density obtainable in the developed image. They not only tend to increase the maximum image density in the case of normally cold-dried images, but also lead to a marked increase in the maximum image density where the images are hot-glazed. Furthermore, the compounds substantially prevent the bronzing effect which, as stated above, has been a common disadvantage in the hot-glazing process.

Improvement in maximum image density is best obtained where the compoundsare included in the emulsion or in the developer or both, and it will be appreciated that by using a compound of the type where the group R is an aryl, substituted aryl; aralkyl or rcycloalkylgroup, an increase in maximum :imagedensity, a:decrease in "bronzing on hot-glazing and'a "decreasein the -fog "level can be rsimultaneously obtained with especially good effect.

The compounds ofthe inventiommay .be prepared by condensing an azideisuch as sodium azide withamustard-oil '(ise. arcompound of the "structure R.NCS where R is a hydrocarbon group) as-describedby .StOlltalldY Strittmatter in the Journal fiir Praktische-rChemie;volume133, pages60-64, and-by Stoll'tand Fr;Henke-Stark in'the'same journal, volumerlZl, pages 261-300. An alternative method for production of the. arylsubstituted zcompounds is'by treatment of an aryl thiosemicarbazide 'with" nitrous acid, as described by "Freund and'Hempel in Berichte der Deutschen Chemischen Gesellschaft, volume 28, at page 7'7.

-'As indicated above, th groupz R in the tantomeric general formulae 'may be :a hydrogen atom or'a hydrocarbon group. "Examples ofthe latter are 'alkyligroupse. g.tmethyl,ethyl..and higher alkyl groups,:.-andeunsaturatedgroups of this-type, e.- g.:-allyl;:-aryl groups; eagephenyl. and naphthylparalkyl' groups,- egg. benzyl;:and cycloalkyl groups, ;e.*g. cyclohexyl. .Suchgroups .may

themselves =be= substituted, zfor example, by halogen atoms OraminogrOups.

The quantity of the compound employed will depend on-the mannerin whichitis brought into "operative contact with the emulsion,-.but..as .a general indication it maybe. statedthat. for incorporation in .an'emulsion l to .5 .or .up to.20 cc. of a 111,000 solution .of..the. compoundv per litre of emulsion is suitable, whilst for incorporation e in a developer, .the optimum vquantity may 'vary .-from1 part of.the..compoundini3,000 to 100,000 .parts of theedeveloperssolution according to the particular compound .used. 'The addition of-the -'compound .to .an-emulsion is .preferably effected after the .digestionof the emulsion, 'as a =final addition before coating.

The following examples illustrate the invention: I

-(-a) Preparation v0]. 1-phenyZ--5-me1ca41to tetrazole.5 .gms. .of .4-phenylethio-semicarbazide, 3.5 ccs. of concentrated hydrochloric acid, and 100005. of water were mixed together and cooled in ice. Themixture was vigorouslystirred and there .was slowly added, in .portions,'.2.1 gms. of solid :sodiumnitrite. A white precipitate 'formed and .the stirring was continued ,for 15 minutes afterall thesodium nitritehadbeen added. The precipitate .was then gfiltered ofi,-washed with water and then immediatelydissolved in 25 cos. of 10% aqueous sodium carbonate solution'- by boiling. The hot solution '-was-'-filtered-and the filtrate allowed to cool. The filtrate-was then acidified with hydrochloric acid, whereupon the tities of .l-phenyl-5-mercapto tetrazole were added. The fog "values and speed values of the separate portions of the emulsion were determined immediately after coating, and again after being kept for several days in a moist warm atmosphere. The original relative speed oi'the control emulsion was taken as'100. The results obtained are. set out in the following Table I.

" Table I Immediately after After keeping Quantity of compound coatmg added periltre of emulswn Relative Fog Relative -Fog speed density speed density 100 0. 05 6O 0. 53 2 cos. of 1:1,000 solution" 92 0105 72 0. 23 4 cos. of 111,000 solution 88 0.04 0.10

It will be observed that the addition'of'4 cos. of a 1 in 1,000 solution of the'compound per litre of emulsion effects a very considerable reduction in the fog value, especially after keeping, and stabilises the speed of the emulsion to a value 'not seriously below that "of'the untreated emulsion.

(0) Use of f-phenyl-5-mercapto tetrazole'ina deoeZoper.--A high'speed panchromatic film was developed for 1 minute at F. in a standard high temperature developing solution. Three tests were made, one with the normal developing solution to. serve as a control, and two others with different added quantities of l-phenyl-5-mercapto tetrazole. The .speed andfog values were determined .and gave the. results .setout in the following Table II. The initial. relative speed of .the control emulsion was againtaken as 100.

Example II .This example-illustrates the results obtainable withvariously substituted 5.-mercapto .tetrazoles included in a photographic silveriodobromide emulsion.

Th results obtained on a .series of tests-are set out in Table IIIwhere in'each line there is given the nature of the l-substituent, the quantity of compoundadded to theemulsion.,,the fog value of a controlsample .of .the emulsion not containing thecompound,:andlastly'the fog value of a test sample containing the stated quantity of the stated compound. The variationin the figures for'the controlitestsis due to the fact that separate controls were effected for eachcase, 'using different batches of emulsion and varying to some extent the emulsion-making conditions.

It will be seen that in every case the compounds are effective in reducing the fog density of the developed images, and this table also illustrates the statement previously made that the substituted aryl; aralkyl and cyclo-alkyl compounds are generally mor effective than the alkyl and unsubstituted compounds of the series, though quite useful improvements are obtained with these latter compounds.

Example III.The use of 1 -methyl 5-mercaptotetrazole in a developer Exposed samples of a photographic printing Paper of the so-called contact type (essentially a silver chloride emulsion) were developed for one minute at 65 F, in a standard metol hydroquinone developer. Two tests are made, on with the normal developing solution and one with a developer containing an addition of 1 part in 10,000 of 1-methyl-5-mercaptotetrazole. The test samples were processed normally and finally glazed on a commercial hot glazing machine. The test sample developed in the absence of l-methyl- 5-mercapto-tetrazole showed a bronzed image having a maximum reflection density of 1.55, while that developed in the presence of l-methyl- 5-mercapto-tetrazole showed a good black image of maximum reflection density 1.95.

Example 4.- Use of I-oz-IlClPhthfUl-5-TIZ6TCdDtO- tetrazole in a developer processed images are shown in the following Table IV:

Table IV Control Test sample Air-dried l. 75 1. 9 Hot-glazed l. 55 2.

It will be noted that the presence of 1-anaphthyl- -mercapto-tetrazole increased the maximum reflection density even in the case of normally dried prints and effected a very substantial increase in the case of prints which were hot-glazed. The hot-glazed control print showed a considerable bronzing eflfect which was substantially elimi- H6 nated in the case of the hot-glazed sample strip containing 1-anaphthy1-5-mercapto-tetrazole.

Example 5.- Use of 1 cycloheacg/l-5-mercaptotetrazole in developer The procedure of Example 3 was followed, precisely except that the samples were of a photographic-paper of the type normally used for making enlargements (i. e. consisting essentially of silver bromide). It was found that the control sample developed in the absence of l-cyclohexyl- S-mercapto-tetrazole showed a somewhat bronzed image having a maximum reflection density of 1.85, while the test sample developed in the presence of the said compound showed a'good black image of maximum reflection density 2.0.

Example 6.Use of 1-phenyl 5 mercapto-tetrazole in a supercoat A silver halide '(mainly silver chloride) photographic emulsion on paper base was coated with a gelatin supercoat containing 10 cos. of a1% solution of 1-phenyl-5-mercapto-tetrazole 'per litre of supercoat solution. A control sample made with a plain gelatin supercoat showed on development a fog density, after keeping, of 0.13, whereas the test sample processed under the same conditions showed a fog density of only 0.014. Moreover, the test sample showed a considerable reduction, compared with the control, in the degree of bronzing of the image effected by hotglazing the prints.

In the following claims, the term 5-mercaptotetrazole and the references to substituents in the 1-position are to be understood in relation to the general tautomeric formulae set out above.

What we claim is: i l

l. A method of improving photographic silver halide emulsions in regard to the quality of images developed therein, which comprises treating the emulsion. at a stage not later than its development, with a small quantity of a 5-mercapto-tetrazole having in the 1-position thereof a group takenirom the class consisting of a hythe class consisting of a hydrogen atom and a hydrocarbon radical.

3. A method of improving photographic silver halide emulsions in regard to the quality of images developed therein, which comprises developing said images with a developer solution which contains a small quantity of a 5-mercaptotetrazole having in the 1-position thereof a group taken from the class consisting of a hydrogen atom and a hydrocarbon radical.

4. A method of improving photographic silver halide emulsions in regard to the quality of images developed therein, which comprises including in the emulsion during its manufacture a 5-mercaptotetrazole containing a cyclic hydrocarbon substituent in the 1-position.

5. A method of improving photographic silver halide emulsions in regard to the quality of images developed therein, which comprises developing said images with a developer solution which contains a small quantity of a S-mercaptotetrazole containing a cyclic hydrocarbon substituent in the 1-position.

6. A method of improving photographic silver halide emulsions in regard to the quality of "images developed therein; which: comprises treating theemulsion; at a stage not'later than its development, with a 5-mercapto-tetrazole containing an aryl substituent inthe'l-position.

'7. A method of improving photographic silver "halide emulsions in regard'to the quality of images developed therein; which comprises de- 'velopingxsaid images Withia developersolution which contains a small quantity of a 5-merca'ptotetrazole containing an aryl substituent in the jlv-po'sition.

8. A silver halide photographic emulsion containing a small quantity of. a S-mercapto-tetra- "zzoleyhaving in the l position thereof a group taken from the class consisting of a hydrogen "atom' andva hydrocarbon radical.

.9. A-photographic element comprising a silver halide photographic emulsion and, in contact therewith, a small quantity of a S-mercapto- *tetrazole havingin' the 1-position thereof agroup "takenirom the class consisting of a hydrogen atom and ahydrocarbon'radical.

T 10. A silver halide photographic emulsion containing a small quantity of a 5-mercapto-tetrazole containing a cyclic hydrocarbon substituent iinthe l-position.

'1'1. A'photographic element comprising a-silver halide photographic emulsion and, in contact therewith, a small-quantity of a 5-mercapto- 'tetrazole containing a cyclic hydrocarbon substituent'in'the l-position.

12. A silver halide photographic emulsion containing a small quantity of "a 5-mercapto-tetra- 'zolecontaining' an'aryl substituent in the I-position.

13. Aphotographic element comprising a silver halide photographic emulsion :andfin contact therewith, a small quantity of a 5-mercapto tetrazole containing 'anaryl substituent in the small quantity .of -a15.-mercapto-tetrazole conence of a small quantity of a .5-mercapto-tetrazole having in the 1-position thereof a group taken from the class consisting of a hydrogen atom and a hydrocarbon radical;

18. A process for the production of photographic prints which comprises developing a latent image contained in a photographic silver halide emulsion'on a paper support, in the presence of a small'quantity of a 5-mercapto-,tetrazole containing a cyclic hydrocarbon substituent in the 1-position.

19. A process for the production of photographic prints which comprises developing a latent image contained in a photographic silver halide emulsion on a paper support, in the presence of a small quantity of a 5-mercapto-tetrazole containing an aryl substituent in the l-position.

20. A process for the production of photographic prints which comprises developing a latent image contained in a photographic silver halide emulsion on a paper support in the presenceof a small 'quantity of a 5-mercaptotetrazolecontaining a hydrocarbon substituent in the 1-position and hot-glazing the paper print 50 obtained.

21. A silver halide photographic emulsion conbon'radical an amount corresponding to that obtained by adding'l to 20 ccs. era 1 to 1000 solution perliter of emulsion.

J OHN'DAVID KENDALL. DOUGLAS JAMES FRY. JAMES DOUGLAS BROOKS. 

